Salmonellosis is caused by a group of gram negative bacteria from the genus Salmonella, and is responsible for approximately 2 million cases of food poisoning annually in the United States. Salmonellosis caused by Salmonella enteritidis (SE) in eggs has been the most important food-born public health Salmonella hazard.
Most outbreaks have been traced to consumption of insufficiently cooked eggs. A number of phage types of SE exist but the majority of outbreaks have been traceable to a single or a few phage types of SE. Human SE food poisoning epidemics have also been reported from many other countries, but the phage types reported have not necessarily been those prevailing in the United States.
Some avian strains of SE are vertically transmitted to the eggs of laying hens. The ovaries, oviduct and isthmus have been identified as sites of vertical transmission. In addition, some observations have pointed to cloacal infection of the egg. It is not known which strains or phage types of SE are vertically transmitted, and genetic or molecular requirements for vertical transmission are not known. Infection of hens with SE has led to colonization of the ceca and of the reproductive organs, usually without disease. In most instances infected hens have continued to lay eggs at normal frequencies. Because of absence of overt disease, infection has been difficult to detect clinically or by serologic diagnostic procedures.
Detection of SE in the hen house, individual hens and/or eggs is a difficult task, with a high degree of uncertainty. In the absence of total eradication of infection, vaccination remains the method of choice for disease control. Because SE does not usually cause disease in hens, there is not a vigorous immune response, and hens remain carriers for long periods, even for life. Therefore, it is presumed that immunization will have to be repeatedly administered. Only live attenuated vaccines given orally can be expected to be efficacious in salmonellosis. Curtiss, R. III, et al., Vet. Microbiol. 37: 397-405 (1993). It is possible that a vaccine strain may have to displace the egg-transmitted strain in the gut and possibly the reproductive tract of hens.
Salmonellosis due to SE is in most instances not a poultry disease but one of the most serious public health hazards worldwide. Prevention of the risk of SE transmission from ingested eggs would save several thousands of lives and would save around $1 billion annually. Thus, a vaccine for poultry against transmission of Salmonella would reduce occurrence of salmonellosis in humans and make a significant contribution to public health worldwide.